Natural gas operations and hydrodesulfurization processes at refineries produce large quantities of hydrogen sulfide (H2S). Some sour gas wells can contain greater than 30% of H2S. H2S is corrosive in nature and impedes the integrity of pipeline. H2S induces hydrate formation and impacts gas production. H2S is toxic, environmental harmful and therefore, it must be neutralized.
In the step of desulfurizing crude oil which is generally conducted presently, heavy naphtha is subjected to hydrofining during crude oil distillation to recover all of the sulfur ingredients contained in the crude oil as hydrogen sulfide. Currently, the predominant process for converting H2S into non-toxic elemental sulfur is the Claus sulphur recovery process. The Claus process includes a number of different steps that are performed to neutralize the toxic H2S. First, the H2S is separated from the host gas stream using amine extraction. Then, the H2S is fed to a Claus unit, where it is converted in the following two steps. The first step is a thermal step in which the H2S is partially oxidized with air. This is done in a reaction furnace at high temperatures (1000-1400° C.). Sulfur is formed, but some H2S remains unreacted, and some SO2 is made. The second step is a catalytic step in which the remaining H2S is reacted with the SO2 at lower temperatures (about 200-350° C.) over a catalyst to make more sulfur. A catalyst is needed in the second step to help the components react with reasonable speed. Unfortunately, the reaction does not go to completion even with an optimal catalyst. For this reason, two or three stages are used, with sulfur being removed between the stages. Inevitably, a small amount of H2S remains in the tail gas and this residual quantity, together with other trace sulfur compounds, is usually dealt with in a tail gas unit.
While the Claus process can yield high conversion rates, there are a number of deficiencies associated with this sulfur recovery process. In particular, the Claus process necessitates an enormous amount of energy because of not only the catalytic reaction of sulfurous acid gas with hydrogen sulfide but also repetitions of heating and condensation. The process has further problems, for example, that the management of sulfurous acid gas is costly. In addition, the process cannot recover the energy contents of H2S and cannot produce highly demanded H2.
Catalysis is the process in which a substance participates in modifying the rate of a chemical transformation of the reactants without being altered or consumed in the end. This substance is known as the catalyst which increases the rate of a reaction by reducing the activation energy. Generally speaking, photocatalysis is a reaction which uses light to activate a substance which modifies the rate of a chemical reaction without being involved itself. The photocatalyst is the substance which can modify the rate of chemical reaction using light irradiation.
A semiconductor photocatalyst has an energy band structure in which the conduction band and the valence band structure are separated by a forbidden band. When a photocatalyst is irradiated with light having energy equal to or higher than a band gap, electrons in the valence band are excited to the conduction band, while holes are generated in the valence band. The electrons excited to the conduction band have higher reducing power than that when the electrons are present in the valence band, and the holes have higher oxidizing power.
Thus, photocatalysis can be in the form of a process that involves light absorption by a semiconductor, particularly, in the form of particulates and generation of excitons to be separated to make redox reactions. The process allows the free-energy positive reaction (a thermodynamically unfavored reaction) to happen utilizing photon energy incident to a reactor (device), which can be utilized for solar energy conversion to chemical energy.
Photocatalytic evolution (generation) of hydrogen using semiconductor powder materials has gained considerable attention because of the importance of solar energy conversion or recovering energy from waste, such as biomass-related organic wastes and hydrogen sulfide. Utilizing solar energy for photocatalysis requires not only extensive absorption in visible light range but also large scale application with low capital cost. The earth-abundant elements are therefore preferred to be the components of the photocatalyst materials. Noble metal nanoparticles are generally good electrocatalyst materials to reduce water/proton to generate hydrogen molecules, and thus efficient photocatalysts can include noble metal nanoparticles on the surface of semiconductor materials. Finding the alternative of noble metals possessing high electrocatalytic activity is still awaited and is desirable.
For high efficient conversion of photocatalytic water splitting, modification of the photocatalyst surface with a cocatalyst is essential because of enhancement in the charge separation creating new metal-semiconductor electronic structure, and in electrocatalytic properties that catalyze the target redox reactions. Many nanomaterials decorated (deposited) on the photocatalyst material, such as Pt, Pt—Pd(S), Au, MoS2, and Ag2S have been reported to enhance photocatalytic hydrogen evolution. For many years, a nickel-based nanoparticle structure is known to be active for photocatalytic water splitting reaction and a moderate size (typically greater than 10 nm) of metallic nickel particles has been considered to be utilized for an active site of hydrogen evolution. A more recent study reported that nickel-thiolate complexes possibly work effectively as hydrogen evolution sites.
Despite these advances, there is still a need for improvements in solar-based hydrogen production technologies and in particular, there is a need for an improved alternative process for the simultaneous conversion of H2S or its containing gases into valuable hydrogen and sulfur. In other words, there is a desire and need for a method for decomposing hydrogen sulphide with a photocatalyst to yield hydrogen and sulfur and which can be put to practical use, thereby making it possible to decompose hydrogen sulfide as a harmful substance with a smaller amount of energy to produce hydrogen and sulfur as useful substances.
This is particularly true for industries that are located in areas that have high annual solar irradiation, such as Saudi Arabia.